From abc6db76520b4675f3fe1bba9c8d57f6d44b6015 Mon Sep 17 00:00:00 2001
From: Paul Duncan <pabs@pablotron.org>
Date: Sat, 2 Sep 2023 23:34:55 -0400
Subject: sha3.[hc]: add cshake128 functions

---
 sha3.c | 352 +++++++++++++++++++++++++++++++++++++++++++++--------------------
 sha3.h |  12 +++
 2 files changed, 255 insertions(+), 109 deletions(-)

diff --git a/sha3.c b/sha3.c
index 71c7ad9..6ad04b7 100644
--- a/sha3.c
+++ b/sha3.c
@@ -344,6 +344,178 @@ void shake256_xof_once(const uint8_t * const src, const size_t src_len, uint8_t
   xof_once(SHAKE256_XOF_RATE, SHAKE256_XOF_PAD, src, src_len, dst, dst_len);
 }
 
+// NIST SP 800-105 utility function.
+static inline size_t left_encode(uint8_t buf[static 9], const uint64_t n) {
+  if (n > 0x00ffffffffffffffULL) {
+    buf[0] = 8;
+    buf[1] = (n >> 56) & 0xff;
+    buf[2] = (n >> 40) & 0xff;
+    buf[3] = (n >> 32) & 0xff;
+    buf[4] = (n >> 24) & 0xff;
+    buf[5] = (n >> 16) & 0xff;
+    buf[6] = (n >> 8) & 0xff;
+    buf[7] = n & 0xff;
+    return 9;
+  } else if (n > 0x0000ffffffffffffULL) {
+    buf[0] = 7;
+    buf[1] = (n >> 56) & 0xff;
+    buf[2] = (n >> 40) & 0xff;
+    buf[3] = (n >> 32) & 0xff;
+    buf[4] = (n >> 24) & 0xff;
+    buf[5] = (n >> 16) & 0xff;
+    buf[6] = (n >> 8) & 0xff;
+    buf[7] = n & 0xff;
+    return 8;
+  } else if (n > 0x000000ffffffffffULL) {
+    buf[0] = 6;
+    buf[1] = (n >> 40) & 0xff;
+    buf[2] = (n >> 32) & 0xff;
+    buf[3] = (n >> 24) & 0xff;
+    buf[4] = (n >> 16) & 0xff;
+    buf[5] = (n >> 8) & 0xff;
+    buf[6] = n & 0xff;
+    return 7;
+  } else if (n > 0x00000000ffffffffULL) {
+    buf[0] = 5;
+    buf[1] = (n >> 32) & 0xff;
+    buf[2] = (n >> 24) & 0xff;
+    buf[3] = (n >> 16) & 0xff;
+    buf[4] = (n >> 8) & 0xff;
+    buf[5] = n & 0xff;
+    return 6;
+  } else if (n > 0x0000000000ffffffULL) {
+    buf[0] = 4;
+    buf[1] = (n >> 24) & 0xff;
+    buf[2] = (n >> 16) & 0xff;
+    buf[3] = (n >> 8) & 0xff;
+    buf[4] = n & 0xff;
+    return 5;
+  } else if (n > 0x000000000000ffffULL) {
+    buf[0] = 3;
+    buf[1] = (n >> 16) & 0xff;
+    buf[2] = (n >> 8) & 0xff;
+    buf[3] = n & 0xff;
+    return 4;
+  } else if (n > 0x00000000000000ffULL) {
+    buf[0] = 2;
+    buf[1] = (n >> 8) & 0xff;
+    buf[2] = n & 0xff;
+    return 3;
+  } else {
+    buf[0] = 1;
+    buf[1] = n & 0xff;
+    return 2;
+  }
+}
+
+// Write prefix for encode_string() to the given buffer.  Accepts the
+// length of the string, in bytes.
+//
+// Returns the length of the prefix, in bytes.
+static inline size_t encode_string_prefix(uint8_t buf[static 9], const size_t num_bytes) {
+  return left_encode(buf, (uint64_t) num_bytes << 3);
+}
+
+typedef struct {
+  size_t prefix_len; // length of bytepad prefix, in bytes
+  size_t pad_len; // number of padding bytes after prefix and data
+} bytepad_lens_t;
+
+// write bytepad() prefix to buffer, then return structure containing
+// the length of the prefix, in bytes, and the total number of
+static inline bytepad_lens_t bytepad_prefix(uint8_t buf[static 9], const size_t data_len, const size_t width) {
+  const size_t prefix_len = left_encode(buf, width);
+  const size_t total_len = prefix_len + data_len;
+  const size_t padded_len = (total_len / width + ((total_len % width) ? 1 : 0)) * width;
+
+  return (bytepad_lens_t) {
+    .prefix_len = prefix_len,
+    .pad_len = padded_len - total_len,
+  };
+}
+
+#define CSHAKE128_XOF_RATE (200 - 2 * 16)
+#define CSHAKE128_XOF_PAD 0x04
+
+_Bool cshake128_xof_absorb(sha3_xof_t * const xof, const uint8_t * const msg, const size_t len) {
+  return xof_absorb(xof, CSHAKE128_XOF_RATE, msg, len);
+}
+
+void cshake128_xof_squeeze(sha3_xof_t * const xof, uint8_t * const dst, const size_t len) {
+  xof_squeeze(xof, CSHAKE128_XOF_RATE, CSHAKE128_XOF_PAD, dst, len);
+}
+
+void cshake128_xof_init(sha3_xof_t * const xof, const cshake_params_t params) {
+  static const uint8_t PAD[CSHAKE128_XOF_RATE] = { 0 };
+
+  if (!params.name_len && !params.custom_len) {
+    // cshake w/o nist prefix and domain is shake
+    shake128_xof_init(xof);
+
+    // FIXME: padding will be wrong on subsequent cshake128_xof_absorb()
+    // calls
+    return;
+  }
+
+  // build nist function name prefix
+  uint8_t name_buf[9] = { 0 };
+  const size_t name_len = encode_string_prefix(name_buf, params.name_len);
+
+  // build custom string prefix
+  uint8_t custom_buf[9] = { 0 };
+  const size_t custom_len = encode_string_prefix(custom_buf, params.custom_len);
+
+  const size_t raw_len = name_len + params.name_len + custom_len + params.custom_len;
+
+  // build bytepad prefix
+  uint8_t bytepad_buf[9] = { 0 };
+  const bytepad_lens_t lens = bytepad_prefix(bytepad_buf, raw_len, CSHAKE128_XOF_RATE);
+
+  // init xof
+  xof_init(xof);
+
+  // absorb cshake prefix and padding
+  (void) cshake128_xof_absorb(xof, bytepad_buf, lens.prefix_len);
+
+  (void) cshake128_xof_absorb(xof, name_buf, name_len);
+  if (params.name_len > 0) {
+    (void) cshake128_xof_absorb(xof, params.name, params.name_len);
+  }
+
+  (void) cshake128_xof_absorb(xof, custom_buf, custom_len);
+  if (params.custom_len > 0) {
+    (void) cshake128_xof_absorb(xof, params.custom, params.custom_len);
+  }
+
+  for (size_t ofs = 0; ofs < lens.pad_len; ofs += sizeof(PAD)) {
+    const size_t len = (lens.pad_len - ofs) < sizeof(PAD) ? lens.pad_len - ofs : sizeof(PAD);
+    (void) cshake128_xof_absorb(xof, PAD, len);
+  }
+}
+
+void cshake128(
+  const cshake_params_t params,
+  const uint8_t * const msg, const size_t msg_len,
+  uint8_t * const dst, const size_t dst_len
+) {
+  if (!params.name_len && !params.custom_len) {
+    // cshake w/o nist prefix and domain is shake
+    shake128_xof_once(msg, msg_len, dst, dst_len);
+    return;
+  }
+
+  // init
+  sha3_xof_t xof;
+  cshake128_xof_init(&xof, params);
+
+  // absorb
+  (void) cshake128_xof_absorb(&xof, msg, msg_len);
+
+  // squeeze
+  cshake128_xof_squeeze(&xof, dst, dst_len);
+}
+
+
 #ifdef SHA3_TEST
 #include <stdio.h> // printf()
 
@@ -1706,70 +1878,6 @@ static void test_shake256_xof_once(void) {
   }
 }
 
-// NIST SP 800-105 utility function.
-static inline size_t left_encode(uint8_t buf[static 9], const uint64_t n) {
-  if (n > 0x00ffffffffffffffULL) {
-    buf[0] = 8;
-    buf[1] = (n >> 56) & 0xff;
-    buf[2] = (n >> 40) & 0xff;
-    buf[3] = (n >> 32) & 0xff;
-    buf[4] = (n >> 24) & 0xff;
-    buf[5] = (n >> 16) & 0xff;
-    buf[6] = (n >> 8) & 0xff;
-    buf[7] = n & 0xff;
-    return 9;
-  } else if (n > 0x0000ffffffffffffULL) {
-    buf[0] = 7;
-    buf[1] = (n >> 56) & 0xff;
-    buf[2] = (n >> 40) & 0xff;
-    buf[3] = (n >> 32) & 0xff;
-    buf[4] = (n >> 24) & 0xff;
-    buf[5] = (n >> 16) & 0xff;
-    buf[6] = (n >> 8) & 0xff;
-    buf[7] = n & 0xff;
-    return 8;
-  } else if (n > 0x000000ffffffffffULL) {
-    buf[0] = 6;
-    buf[1] = (n >> 40) & 0xff;
-    buf[2] = (n >> 32) & 0xff;
-    buf[3] = (n >> 24) & 0xff;
-    buf[4] = (n >> 16) & 0xff;
-    buf[5] = (n >> 8) & 0xff;
-    buf[6] = n & 0xff;
-    return 7;
-  } else if (n > 0x00000000ffffffffULL) {
-    buf[0] = 5;
-    buf[1] = (n >> 32) & 0xff;
-    buf[2] = (n >> 24) & 0xff;
-    buf[3] = (n >> 16) & 0xff;
-    buf[4] = (n >> 8) & 0xff;
-    buf[5] = n & 0xff;
-    return 6;
-  } else if (n > 0x0000000000ffffffULL) {
-    buf[0] = 4;
-    buf[1] = (n >> 24) & 0xff;
-    buf[2] = (n >> 16) & 0xff;
-    buf[3] = (n >> 8) & 0xff;
-    buf[4] = n & 0xff;
-    return 5;
-  } else if (n > 0x000000000000ffffULL) {
-    buf[0] = 3;
-    buf[1] = (n >> 16) & 0xff;
-    buf[2] = (n >> 8) & 0xff;
-    buf[3] = n & 0xff;
-    return 4;
-  } else if (n > 0x00000000000000ffULL) {
-    buf[0] = 2;
-    buf[1] = (n >> 8) & 0xff;
-    buf[2] = n & 0xff;
-    return 3;
-  } else {
-    buf[0] = 1;
-    buf[1] = n & 0xff;
-    return 2;
-  }
-}
-
 static void test_left_encode(void) {
   static const struct {
     const char *name;
@@ -1840,14 +1948,6 @@ static void test_left_encode(void) {
   }
 }
 
-// Write prefix for encode_string() to the given buffer.  Accepts the
-// length of the string, in bytes.
-//
-// Returns the length of the prefix, in bytes.
-static inline size_t encode_string_prefix(uint8_t buf[static 9], const size_t num_bytes) {
-  return left_encode(buf, (uint64_t) num_bytes << 3);
-}
-
 static void test_encode_string_prefix(void) {
   static const struct {
     const char *name;
@@ -1904,24 +2004,6 @@ static void test_encode_string_prefix(void) {
   }
 }
 
-typedef struct {
-  size_t prefix_len; // length of bytepad prefix, in bytes
-  size_t pad_len; // number of padding bytes after prefix and data
-} bytepad_lens_t;
-
-// write bytepad() prefix to buffer, then return structure containing
-// the length of the prefix, in bytes, and the total number of
-static inline bytepad_lens_t bytepad_prefix(uint8_t buf[static 9], const size_t data_len, const size_t width) {
-  const size_t prefix_len = left_encode(buf, width);
-  const size_t total_len = prefix_len + data_len;
-  const size_t padded_len = (total_len / width + ((total_len % width) ? 1 : 0)) * width;
-
-  return (bytepad_lens_t) {
-    .prefix_len = prefix_len,
-    .pad_len = padded_len - total_len,
-  };
-}
-
 static void test_bytepad_prefix(void) {
   static const struct {
     const char *name;
@@ -1977,27 +2059,78 @@ static void test_bytepad_prefix(void) {
   }
 }
 
-void cshake128(
-  const uint8_t * const nist_name, const size_t nist_name_len,
-  const uint8_t * const domain, const size_t domain_len,
-  const uint8_t *msg, const size_t msg_len,
-  uint8_t * const dst, const size_t dst_len
-) {
-  if (nist_name_len || domain_len) {
-    // TODO
-  } else {
-    // without nist prefix and domain, cshake128 is equivalent to
-    // shake128()
-
-    // init
-    sha3_xof_t xof;
-    shake128_xof_init(&xof);
+static void test_cshake128(void) {
+  static const struct {
+    const char *test_name; // test name
+    const uint8_t name[256]; // nist function name
+    const size_t name_len; // nist function name length
+    const uint8_t custom[256]; // custom name
+    const size_t custom_len; // custom name length
+    const uint8_t msg[256]; // test message
+    const size_t len; // test message length
+    const uint8_t exp[32]; // expected hash
+  } tests[] = {{
+    // src: https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Standards-and-Guidelines/documents/examples/cSHAKE_samples.pdf
+    .test_name = "cSHAKE Sample #1",
+    .custom = "Email Signature",
+    .custom_len = 15,
+    .msg = { 0, 1, 2, 3 },
+    .len = 4,
+    .exp = {
+      0xC1, 0xC3, 0x69, 0x25, 0xB6, 0x40, 0x9A, 0x04,
+      0xF1, 0xB5, 0x04, 0xFC, 0xBC, 0xA9, 0xD8, 0x2B,
+      0x40, 0x17, 0x27, 0x7C, 0xB5, 0xED, 0x2B, 0x20,
+      0x65, 0xFC, 0x1D, 0x38, 0x14, 0xD5, 0xAA, 0xF5,
+    },
+  }, {
+    // src: https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Standards-and-Guidelines/documents/examples/cSHAKE_samples.pdf
+    .test_name = "cSHAKE Sample #2",
+    .custom = "Email Signature",
+    .custom_len = 15,
+    .msg = {
+      0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
+      0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F,
+      0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E, 0x2F,
+      0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A, 0x3B, 0x3C, 0x3D, 0x3E, 0x3F,
+      0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F,
+      0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5A, 0x5B, 0x5C, 0x5D, 0x5E, 0x5F,
+      0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F,
+      0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7A, 0x7B, 0x7C, 0x7D, 0x7E, 0x7F,
+      0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8A, 0x8B, 0x8C, 0x8D, 0x8E, 0x8F,
+      0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9A, 0x9B, 0x9C, 0x9D, 0x9E, 0x9F,
+      0xA0, 0xA1, 0xA2, 0xA3, 0xA4, 0xA5, 0xA6, 0xA7, 0xA8, 0xA9, 0xAA, 0xAB, 0xAC, 0xAD, 0xAE, 0xAF,
+      0xB0, 0xB1, 0xB2, 0xB3, 0xB4, 0xB5, 0xB6, 0xB7, 0xB8, 0xB9, 0xBA, 0xBB, 0xBC, 0xBD, 0xBE, 0xBF,
+      0xC0, 0xC1, 0xC2, 0xC3, 0xC4, 0xC5, 0xC6, 0xC7,
+    },
+    .len = 200,
+    .exp = {
+      0xC5, 0x22, 0x1D, 0x50, 0xE4, 0xF8, 0x22, 0xD9,
+      0x6A, 0x2E, 0x88, 0x81, 0xA9, 0x61, 0x42, 0x0F,
+      0x29, 0x4B, 0x7B, 0x24, 0xFE, 0x3D, 0x20, 0x94,
+      0xBA, 0xED, 0x2C, 0x65, 0x24, 0xCC, 0x16, 0x6B,
+    },
+  }};
 
-    // absorb (note: ignoring error message here)
-    (void) shake128_xof_absorb(&xof, msg, msg_len);
+  for (size_t i = 0; i < sizeof(tests) / sizeof(tests[0]); i++) {
+    const cshake_params_t params = {
+      .name = tests[i].name,
+      .name_len = tests[i].name_len,
+      .custom = tests[i].custom,
+      .custom_len = tests[i].custom_len,
+    };
+
+    // run
+    uint8_t got[32];
+    cshake128(params, tests[i].msg, tests[i].len, got, sizeof(got));
+
+    // check
+    if (memcmp(got, tests[i].exp, sizeof(got))) {
+      fprintf(stderr, "test_cshake128(\"%s\") failed, got:\n", tests[i].test_name);
+      dump_hex(stderr, got, 32);
 
-    // squeeze
-    shake128_xof_squeeze(&xof, dst, dst_len);
+      fprintf(stderr, "exp:\n");
+      dump_hex(stderr, tests[i].exp, 32);
+    }
   }
 }
 
@@ -2021,6 +2154,7 @@ int main(void) {
   test_left_encode();
   test_encode_string_prefix();
   test_bytepad_prefix();
+  test_cshake128();
   printf("ok\n");
 }
 
diff --git a/sha3.h b/sha3.h
index c241758..56e151c 100644
--- a/sha3.h
+++ b/sha3.h
@@ -36,6 +36,18 @@ _Bool shake256_xof_absorb(sha3_xof_t * const xof, const uint8_t * const m, const
 void shake256_xof_squeeze(sha3_xof_t * const xof, uint8_t * const dst, const size_t dst_len);
 void shake256_xof_once(const uint8_t * const src, const size_t src_len, uint8_t * const dst, const size_t dst_len);
 
+typedef struct {
+  const uint8_t * const name; // NIST function name
+  const size_t name_len; // length of NIST function name, in bytes
+  const uint8_t * const custom; // customization string
+  const size_t custom_len; // length of customization string, in bytes
+} cshake_params_t;
+
+void cshake128_xof_init(sha3_xof_t * const xof, const cshake_params_t params);
+_Bool cshake128_xof_absorb(sha3_xof_t * const xof, const uint8_t * const msg, const size_t len);
+void cshake128_xof_squeeze(sha3_xof_t * const xof, uint8_t * const dst, const size_t len);
+void cshake128(const cshake_params_t params, const uint8_t * const msg, const size_t msg_len, uint8_t * const dst, const size_t dst_len);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */
-- 
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